The present invention relates to a process for producing a laminated sheet comprising an alumina fiber precursor spun out from a spinning solution containing an aluminum compound. More particularly, it relates to a process for producing a laminated sheet comprising an alumina fiber precursor having a uniform basis weight throughout. Alumina fiber sheet obtained by calcining the said laminated sheet has excellent refractory and heat insulating properties as well as high mechanical strength and chemical stability even under high temperatures and are used as a high-temperature refractory/heat insulator, high-temperature cushioning medium and such.
It is known to produce alumina fiber by first forming an alumina fiber precursor by spinning from a spinning solution, and then calcining the said precursor. This method is especially suited for producing alumina fiber whose alumina content exceeds 65% by weight, such the production that the conventional melt fiber-forming method is inapplicable. The spinning solution used in this method is principally comprising an aluminum compound and contains small amounts of various adjuvants. The adjuvants include those which become the structural elements of the finally produced alumina fiber, such as metal compounds, and those which serve for adjusting the properties of the spinning solution, such as water-soluble polymeric compounds. For example, a spinning solution prepared by adding silica sol and polyvinyl alcohol to a basic aluminum chloride solution formed by dissolving aluminum in hydrochloric acid is used.
Blowing method and spindle method utilizing centrifugal force are known for spinning out an alumina fiber precursor from a spinning solution, but usually blowing method is used. According to this blowing method, the spinning solution is supplied into a high-speed spinning air stream from a nozzle, the-supplied spinning solution being drawn out in the spinning air stream, deprived of moisture and solidified to form an alumina fiber precursor.
The thus formed alumina fiber precursor is amassed to form an alumina fiber precursor sheet having a specified basis weight, i.e., a specified weight per unit area. Although the constituent alumina fiber precursor has flexibility, the precursor sheet itself is low in fiber strength and also unstable as it contains structural water and/or additives in fiber, so that usually this precursor sheet, can not be offered as a commercial product in the form as it is. Therefore, it is necessary to calcine the alumina fiber precursor sheet to form an alumina fiber sheet having high crystallinity while maintaining a stable oxide state. It is also possible to obtain an alumina fiber sheet with even higher mechanical strength by needling the precursor sheet before calcining. (See U.S. Pat. Nos. 4,752,515, 4,931,239 and 5,104,713).
As means for producing an alumina fiber precursor sheet having a specified basis weight (fiber weight per unit area or basis area weight) by amassing the alumina fiber precursor, a method is known in which the alumina fiber precursor in the spinning air stream is fallen and stacked on an accumulator until a sheet with a specified basis weight is formed. For example, the alumina fiber precursor is fallen and stacked on a rotating endless belt, and the alumina fiber precursor sheet formed by stacking the said precursor is successively tugged out from the endless belt.
A method is also known in which the alumina fiber precursor carried in the spinning air stream is fallen and stacked on an accumulator to form a thin lamina sheet which is far smaller in thickness than the sheet to be formed having a specified basis weight, and this lamina sheet, in the next step, is wound round a number of times until forming the sheet with a specified basis weight. In a typical example of this method, a spinning air stream containing the alumina fiber precursor is let impinge almost at right angles against a rotating endless belt of the type which allows easy passage of air, such as a belt made of (metal) wire mesh (net). The spinning air stream is allowed to pass through the endless belt, but the alumina fiber precursor is caught and amassed on the endless belt to form a lamina sheet. This lamina sheet of alumina fiber precursor is pulled apart from the endless belt and wound around a rotator in whatever layers until forming a sheet having a specified basis weight. Then the roll of the laminated sheet on the rotator is cut into sections, and subjected to the ensuing steps such as calcining.
According to the above method, although capture and amassing of the alumina fiber precursor from the spinning air stream is easy, the sheet forming operations are complicated as they are batch type, and further, since the length of the sheet that can be treated depends on the circumferential length of the rotator, it is impossible to obtain sheets of all required lengths.
A further problem of the said conventional method is that the formed alumina fiber precursor sheet is non-uniform in basis weight along the width thereof, the basis weight being particularly small at both end portions of the sheet. This is for the reason that when the alumina fiber precursor is fallen from the spinning air stream and stacked on an accumulator, the precursor does not stack uniformly along the whole width of the accumulator, and most remarkably the stacking at both ends in the width direction is relatively small.
That the basis weight of the alumina fiber precursor sheet is non-uniform along the width thereof, particularly small at both ends, signifies corresponding variation of the basis weight of the calcined alumina fiber sheet in its width direction. An alumina fiber sheet as a commercial product is required to be uniform in basis weight in its entirety, so that both end portions in the width direction where the basis weight is smaller than the specified value must be cut out rather overly, which results in a reduced yield of the alumina fiber sheet. Also, even if both end portions are cut out, the sheet would have to be disposed off as a substandard product if there still exists a portion where the basis weight is outside the specified range.
In recent years, attention is focused on application of alumina fiber sheets to such areas as holding means for exhaust gas cleaning systems, heat-resistant filters and the like, and in such uses, higher precision of sheet thickness than in the conventional uses is required. For example, in the internal combustion engines, as a measure for disposal of exhaust gas, a cleaning system having a honeycomb catalyst housed in a casing is provided in the exhaust gas passage. For securely holding such honeycomb catalyst in the catalyst casing, it is necessary to wind a holding mat for catalyst holding member around the honeycomb catalyst to as much a uniform thickness as possible and house this catalyst in the casing so that it will be closely secured to the inside wall of the casing by the restoring force of the holding member. Such a holding member is preferably a fiber sheet which is proof against fiber deterioration and capable of maintaining an appropriate surface pressure even under high temperatures. Japanese Patent Application Laid-Open (KOKAI) No. 7-286514, for instance, teaches that among alumina fiber sheets, the one produced by laminating alumina fiber having a composition of Al2O3:SiO2=70-74:30-26 (by weight) and needling the laminate is especially preferred.
As a result of the present inventors' earnest studies to solve the above problem, it has been found that by folding the thin lamina sheet of alumina fiber precursor by a predetermined width while stacking the folded sheet and continuously moving the stacking sheet in the direction orthogonal to the folding direction, the obtained alumina fiber precursor sheet has uniform basis weight along the full width thereof.
The present invention has been attained on the basis of the above finding.